e Staphylococcal enterotoxins (SEs) produced by Staphylococcus aureus have superantigenic and emetic activities, which cause toxic shock syndrome and staphylococcal food poisoning, respectively. Our previous study demonstrated that the sequence of SET has a low level of similarity to the sequences of other SEs and exhibits atypical bioactivities. Hence, we further explored whether there is an additional SET-related gene in S. aureus strains. One SET-like gene was found in the genome of S. aureus isolates that originated from a case of food poisoning, a human nasal swab, and a case of bovine mastitis. The deduced amino acid sequence of the SET-like gene showed 32% identity with the amino acid sequence of SET. The SET-like gene product was designated SElY. In the food poisoning and nasal swab isolates, mRNA encoding SElY was highly expressed in the early log phase of cultivation, whereas a high level of expression of this mRNA was found in the bovine mastitis isolate at the early stationary phase. To estimate whether SElY has both superantigenic and emetic activities, recombinant SElY was prepared. Cell proliferation and cytokine production were examined to assess the superantigenic activity of SElY. SElY exhibited superantigenic activity in human peripheral blood mononuclear cells but not in mouse splenocytes. In addition, SElY exhibited emetic activity in house musk shrews after intraperitoneal and oral administration. However, the stability of SElY against heating and pepsin and trypsin digestion was different from that of SET and SEA. From these results, we identified SElY to be a novel staphylococcal emetic toxin. Staphylococcus aureus produces a variety of exotoxins, including staphylococcal enterotoxins (SEs) and toxic shock syndrome toxin 1 (TSST-1) (1). These toxins are superantigens, which have the ability to stimulate a large repertoire of the V␤ elements of T cell receptor (TCR)-bearing T cells. SEs are also emetic toxins causing staphylococcal food poisoning in humans, although the mechanism of SE-induced emesis has not been entirely verified (2). Due to these properties, SEs are assumed to be a menace to public health. Five major serological types (SEA to SEE) have been characterized (2), and new types of SE-related toxins (SEG to SEI, SElJ, SEK to SET, SElU, SElV, and SElX) have recently been reported (2-8). Moreover, superantigen-related genes, such as staphylococcal superantigen-like protein (SSL) genes (ssl1 to ssl26), were discovered during determination of the complete genome sequences of several S. aureus strains (9-11). It has been recognized that the superantigen genes are associated with mobile genetic elements (MGEs), such as pathogenicity islands, prophages, or plasmids (5,(11)(12)(13)(14). This fact implies that these superantigen genes move among S. aureus strains by horizontal transfer and that such MGEs play an important role in the evolution of S. aureus as a pathogen.We have reported that SET shows mitogenicity to human T cells and requires major histocompatibility complex (MHC) cla...

e Molecular characterization of isolates from staphylococcal food poisoning (SFP) outbreaks in Japan showed that the dominant lineage causing SFP outbreaks is clonal complex 81 (CC81), a single-locus variant of sequence type 1, coagulase type VII, positive for sea and/or seb, and positive for seh. Among various CC lineages producing staphylococcal enterotoxin A, CC81 showed the highest toxin productivity. Staphylococcal food poisoning (SFP), one of the most common food-borne diseases, results from the consumption of foods containing toxic amounts of staphylococcal enterotoxins (SEs) (1-4). SFP is associated with toxigenic Staphylococcus aureus strains that produce one or more members of a family of genes encoding heat-stable SEs. Recently, a superfamily of more than 23 different SEs and SE-like toxins (SEls) was studied for their biological activities (4-8). These bacterial toxins are also known as pyrogenic superantigens that stimulate polyclonal T-cell proliferation and can potentially cause toxic shock syndrome (1-4). The genes for SEs and SEls are harbored by various mobile genetic elements and/or genomic islands, including prophages, plasmids, S. aureus pathogenicity islands (SaPIs), and Sa␤. To date, in addition to the five classical types of SEs (SEA through SEE), 18 new types of SEs and SEls (SEG through SElX) have been described (4-8). Our recent study confirmed the emetic potential of SElK, SElL, SElM, SElN, SElO, SElP, and SElQ in the monkey, and these SEls were renamed SEK, SEL, SEM, SEN, SEO, SEP, and SEQ, respectively (9). Comparing SEs and SEls, SEA is considered the most important SE causing SFP. S. aureus is a common commensal bacterium of the skin and mucosal surfaces of humans, with estimates of 20% persistent and 60% intermittent colonization (10). Food handlers carrying enterotoxin-producing S. aureus in their nasal cavities or on their skin are important sources of food contamination during the cooking process (3). Contamination with S. aureus is believed to be associated primarily with improper handling of cooked or processed foods with improper storage under conditions that allow the growth of S. aureus and the production of SEs. Reports concerning the molecular epidemiology and genetic diversity of the isolates from SFP outbreaks are limited when clinical isolates are compared. In this investigation, we ex-

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Staphylococcal enterotoxins (SEs) produced by Staphylococcus aureus are the most recognizable causative agents of emetic food poisoning in humans. New types of SEs and SE-like (SEl) toxins have been reported. Several epidemiological investigations have shown that the SEs and SEl genes, particularly, SEK, SEL, SEM, SEN and SEO genes, are frequently detected in strains isolated from patients with food poisoning. The purpose of the present study was to evaluate the emetic activity of recently identified SEs using a small emetic animal model, the house musk shrew. The emetic activity of these SEs in house musk shrews was evaluated by intraperitoneal administration and emetic responses, including the number of shrews that vomited, emetic frequency and latency of vomiting were documented. It was found that SEs induce emetic responses in these animals. This is the first time to demonstrate that SEK, SEL, SEM, SEN and SEO possess emetic activity in the house musk shrew.